The present invention relates to apparatus for reading digital data recorded on a record medium such as a floppy disk, hard magnetic disk, magnetic tape or the like. The invention is specifically and advantageously designed for use in a read circuit for a floppy disk drive.
Apparatus for reading pre-recorded digital data from a record medium, such as a floppy disk, normally comprises a read transducer, such as a magnetic head, for generating an electrical signal corresponding to the recorded data; an amplifier for amplifying this electrical signal; suitable filters for modifying the electrical signal; and a decoder for generating a data stream from the filtered signal. In circuits of this kind, two types of filters are sometimes used: (1) a so-called "equalizing filter" for enhancing the recording signal to compensate for the finite, real world constraints in writing and reading from recorded media and (2) a so-called "Gaussian filter" for removing white noise and other high frequency noise components from the recording signal.
Much attention has been directed to the development of equalizing filters for reading recorded data from magnetic media. R. C. Schneider describes "An Improved Pulse-Slimming Method For Magnetic Recording" in IEEE Transactions on Magnetics, Vol. MAG-11, No. 5, Sept. 1975. This article discloses a basic pulse-slimming building block that enables the leading and trailing edge of an isolated read pulse to be slimmed independently. The first stage of this building block (shown in FIG. 1) slims the trailing edge of the pulse by adding a portion of the first derivative. The second stage slims the leading edge of the pulse by subtracting a portion of the first derivative. This building block, combined with an integrator to convert peaks the zero-crossings, equalizes the phase and amplitude of the recording channel. The frequency response of this circuit is shown in FIG. 8.
C. E. Schlaepfer discloses linear waveform filters in "Signal Processing For Increased Bit Densities In Digital Magnetic Recording", published by I.B.M. Corp., San Jose, Calif. In this paper, Schlaepfer discusses the problem of degeneration of readback signals caused by interaction of adjacent pulses. As shown in FIG. 2 of the paper, increasing the data packing density beyond the condition where the bit period is less than half the pulse basewidth results in detrimental interaction between adjacent pulses. For two adjacent "1's" surrounded by "0's" the familiar peak-shift phenomenon occurs. This peak-shift may be viewed as a linear superposition of individual basic pulses so that the analysis of the readback signal is simplified to the treatment of a single representative pulse.
In order to permit higher data packing densities without pulse interaction, Schlaepfer provides an example of a pulse-slimming filter (FIG. 7). Like the "building block" of Schneider, this filter amplifies the higher frequency components of the read signal. This is accomplished while fulfilling the requirement that the filter have a linear time delay; that is, that it provide the same phase delay at all frequencies. However, this filter has two main disadvantages: (1) it has a 12 dB/octave gain rise which is too much of an increase in amplitude at the higher frequency components, and (2) it uses a passive network which creates a lower frequency pole, resulting in the necessity of adding a differentiator to cancel this pole.